The electronic spectrum of [Pt(tpy)Cl](+) (tpy = 2,2’:6’,2"-terpyridine) is influenced dramatically by intermolecular stacking interactions in solution and in the solid state. The crystal structure of [Pt(tpy)Cl]ClO4 (monoclinic, P2(1)/c (No. 14); a = 7.085(2), b = 17.064(5), c = 26.905(8) Angstrom; beta = 90.0(1)degrees; Z = 8) consists of discrete Pt-2 units (Pt-Pt = 3.269(1) Angstrom) arranged along an infinite tpy-pi stack (spacing similar to 3.35 Angstrom. Variable-temperature and concentration studies of the absorption and emission spectra of [Pt(tpy)Cl](+) suggest that similar metal-metal and ligand-ligand interactions persist in the solution phase. The high concentration, low-temperature emission spectrum (5:5:1 ethanol:methanol:DMF) reveals a 740-nm band indicative of M-M oligomerization, a 650-nm band attributable to tpy pi-pi interactions, and a 470-nm band characteristic of mononuclear [Pt(tpy)Cl](+) pi-pi* emission. Concentration-dependent absorption spectra were fit to a "two-dimer" model, yielding equilibrium constants for the formation of Pt-Pt-, and tpy-tpy-bound dimers of 1.3(1) x 10(3) and 1.0(1) x 10(3) M(-1), respectively, in 0.1 M aqueous NaCl. The low temperature solid-state luminescence of [Pt(tpy)Cl](+) is assigned to a (3)(MMLCT) (MMLCT = metal-metal-to-ligand charge transfer) transition.